ORIGIN OF VASCULAR TISSUES 47 



anlage would be demonstratedly not vascular anlages at all, but arti- 

 facts or 'Mayer-Lewis anlages/ and no study by slide or section could 

 establish their right to serious consideration. 



It is of interest in this connection to note that McClure (33, 

 34, 36) has been able actually to inject a discrete lymphatic 

 anlage — the sub-ocular lymph sac in the trout — independently 

 of the main systemic lymphatics, veins, and arteries, yet it is 

 inconceivable that these latter vessels, as revealed in section, 

 could be regarded as non-vascular spaces. 



In defense of the injection method, E. R. Clark (8) has at- 

 tempted to show by tests the adequacy of injection as com- 

 pared with serial sections, which are often relied on by those 

 who believe in the local origin of endothelium. He injected 

 small lymphatics, drew their outlines, then sectioned the tissue 

 and reconstructed the vessels. He found that the lines formerly 

 occupied by the continuous vessels were occupied by series of 

 isolated, black, bead-like areas. Doubtless one does well to 

 admit that shrinkage actually takes place in preserved tissues, 

 but doubtless also there is no other case in the literature where 

 the series of isolated spaces comparable to these are individ ually 

 figured in solid black. If one should combine complete injec- 

 tion with serial sectioning, and find lines of discontinuous 

 injected spaces he would probably not commit the error of regard- 

 ing them as always having been out of continuity. In all proba- 

 bility the only recorded case of an injected isolated vascular 

 space, besides those figured by Clark, is that of McClure's 

 subocular sac; this was observed in the living condition to be 

 independent, and was injected directly. 



As already stated, those who believe endothelium to form 

 entirely by proliferation of a small, restricted yolk-sac anlage 

 have been strengthened in their belief by observation of the 

 sprouting of living endothelium. On the other hand, direct 

 observation of the mesenchymal synthesis of endothelium is by 

 no means wanting. As early as 1885 Wenckebach (77) described 

 with great accuracy the remarkable 'Bewusstsein' with which 

 single mesenchyme cells migrate by amoeboid movement to 

 arrange themselves into endothelial tubes on the teleost yolk- 



